The present invention relates generally to very high speed discrete multi-carrier data transmission systems. More particularly, a mechanism for reducing the impact of radio frequency (RF) interference in multi-carrier transmission schemes is disclosed.
The Alliance For Telecommunications Industry Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has recently finalized a discrete multi-tone based standard for the transmission of digital data over Asymmetric Digital Subscriber Lines (ADSL). The standard is intended primarily for transmitting video data over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard and is incorporated herein by reference. Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of at least 6 million bits per second (i.e., 6+ Mbit/s) over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 "tones" or "sub-channels" or that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied. However when modulation is performed efficiently using an Inverse Fast Fourier Transform (IFFT), typical values for the number of available sub-channels (tones) are integer powers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels.
The Asymmetric Digital Subscriber Lines standard also defines the use of a reverse signal at a data rate in the range of 16 to 800 Kbit/s. The reverse signal corresponds to transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term Asymmetric Digital Subscriber Line comes from the fact that the data transmission rate is substantially higher in the downstream direction than in the upstream direction. This is particularly useful in systems that are intended to transmit video programming or other video signals to a remote location over the telephone lines. By way of example, one potential use for such systems allows residential customers to obtain video information such as movies over the telephone lines or cable rather than having to rent video cassettes. Another potential use is in video conferencing.
At the time of this writing, ATIS has begun work on the next generation subscriber line based transmission system, which is referred to as the VDSL (Very High-Speed Digital Subscriber Line) standard. The VDSL standard is intended to facilitate transmission rates of at least 12.98 Mbit/s and preferably 25.96 or 51.92 Mbit/s in the downstream direction. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a short range system, which is referred to as Fiber To The Curb (FTTC). The transmission medium to the customer premise of both these systems is standard unshielded twisted pair (UTP) telephone lines.
A number of multi-carrier modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). One proposed multi-carrier solution utilizes discrete multi-tone (DMT) signals in a system that is similar in nature to the ADSL standard. Other proposed modulation schemes include carrierless amplitude and phase modulated (CAP) signals; discrete wavelet multi-tone modulation (DWMT); and OFDM which is a simplified version of DMT. In order to achieve the data rates required by VDSL/FTTC, the transmission bandwidth must be significantly broader than the bandwidth contemplated by the ADSL. By way of example, the discrete multi-tone system adopted for ADSL applications utilizes a transmission bandwidth on the order of 1.1 MHz, while bandwidths on the order of 10 MHz are being contemplated for VDSL/FTTC applications. In one proposed DMT system for VDSL/FTTC applications, the use of 256 "tones" or "sub-channels" that are each 43.125 kHz wide is contemplated.
The use of these wide band multi-carrier modulation approaches have some inherent obstacles that must be overcome. One particular problem relates to radio frequency interference. As is well known to those skilled in the art, amateur radio operators utilize several relatively narrow frequency bands in the 1 MHz to 12 MHz frequency range. By way of example, the following approximate frequency bands are reserved for amateur radio operators use: 1.8 to 2.0 MHz; 3.5 to 4.0 MHz; 7.0 to 7.3 MHz and 10.1 to 10.15 MHz. Thus, it is important that any subscriber line based modulation scheme not output significant interference in those frequency bands and, of course, the system must be arranged to handle particularly high levels of interference in those frequency ranges. The present invention provides a mechanism for handling such matters.